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24V 3A output required from HP Z-book laptop.

Discussion in 'General Electronics Discussion' started by Pete Fowler, Feb 23, 2015.

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  1. Pete Fowler

    Pete Fowler

    Feb 23, 2015
    Hi everyone, my first post here.
    I work in marine electronics, and one of the components I service on vessels needs to be connected to my laptop via serial port and at the same time needs power. The component is sometimes situated away from any power sources. It would be very handy to have a 24V 3A output from my laptop. It's a HP Z-book 15, realizing that usb would not provide enough power I was thinking of using the thunderbolt port to power the component. Does anyone know if this would be possible?
    Thanks in advance.
  2. Gryd3


    Jun 25, 2014
    Very very unlikely...
    24V @ 3A is quite a bit of power for a laptop to be able to provide...
    Even if this were possible, it would cut the battery life at least half while it were active, and that would require a lot of very heavy modifications.

    Edit: The port you are thinking of using maxes out at about 18V and just shy of 600mA ... no amount of magic will let you pull 6 times the power from this port.
  3. Pete Fowler

    Pete Fowler

    Feb 23, 2015
    Thanks for the reply,
    I would only need the output for a short period of time, approximately 5-10 minutes, and with a battery capacity of 5.1 Ah battery life shouldn't be an issue.
    18V would suffice but I wasn't aware of the current limit for thunderbolt.
    Any idea of another way to acheive this? I realise a seperate battery seems the logical way to go but for weight saving purposes and conveniance I would really like to use the laptop battery.
  4. Gryd3


    Jun 25, 2014
    You would most likely need to modify the battery pack to provide an auxiliary output... the problem here is that if any additional device pulls too much instantaneous current, it could cause the voltage to drop and the laptop to crash... Additionally, laptop batteries and the laptops have numerous safety mechanisms in place to protect the battery from excessive discharge.
    I would strongly advise against it... but if you are stubborn like me, you will most likely want to try anyway.
    KrisBlueNZ likes this.
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I second everything that Gryd3 said. Your load needs 24V at 3A which is 72W. It will have to come from a boost converter, which might be 80% efficient, so its input power will be 90W. That's at least twice what the rest of the laptop draws from the battery, so it will cut your run time to around 30% of normal.

    You can't draw anything like 90W through any standard port, so you would have to hack the battery pack to add a resettable fuse and a socket. If the laptop battery is 14V for example, that corresponds to 6.5A. Probably more than a standard adapter plug/socket is designed for, but you should use a non-standard connector anyway.

    Modifying the battery pack in any way will certainly void any warranty, and likely to be a perceived (even if not actual) safety risk for explosion, fire and chemical leakage, and maybe others. Those things usually happen during charging, but no one is going to guarantee that they won't happen during discharge, especially if you're drawing three times the normal amount of current from it.

    Finally, drawing energy from the battery "behind the laptop's back" could cause problems with the laptop's ability to monitor the amount of energy in the battery, and this could lead to undercharging or overcharging.

    So in other words, I think it would be very unwise to try it. Perhaps you could use an external 24V SLA/VRLA battery to power the external load and the laptop (using a 24V to 19V buck converter) and remove the laptop's internal battery to save a bit of weight.
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